Tuning circuit



June 27, 1939. P. WARE: 2,163,646

TUNING CIRCUIT Filed Aug. 8, 1956 2 Sheets-Sheet l Z-U 5.0 4-0 6 0 6.a7o da 2o, Za-

.200 Iil ATTORNEY v June 27, 1939. P. WARE `'lUIl-IIK CIRCUIT Filed Augs, 1936 2 Sheets-Sheet 2 INVENTOR Haw! /fe' BY l,

rij ATTORNEY Patented June 27, 1939 vUNITED STATES PATENT oFFlcE TUNINGCIRCUIT vPaul Ware, Indianapolis, Ind., assignor to P. R.

Mallory & Co., Inc., Indianapolis, Ind., a corporation of DelawareApplication August 8, 1936, Serial No. 94,928

5 Claims.

This invention relates to improvements in radio tuning circuits havingvariable inductances.

Certain of its objects are to improve the efficiency and range of suchcircuits.

Anotherobject is to furnish a means for holding in step, or track, theseveral circuitsrequired in superheterodyne pre-selection.

' Other objects of theinventionwill be apparent from the followingdescription drawings taken in connection claims.

and accompanying with the appended The invention comprises the featuresofconstruction, combination of elements,` arrangement of parts, andmethods of manufacture and. op-era- Vtion referred to above or whichwillk be brought out and exemplified in the disclosure hereinafter setforth including'the illustrations in the drawings.

Figure 1 illustrates a portion of a tuning circuit Vemploying a variableinductance coil;

Figure 2 is a graph having curves representing the Q of certain coilassemblies over a range of frequencies;

Figure 3 illustrates conventional variable condenser tuning;

. Figure i shows a variable arrangement;`

inductance tuning Figure 5 shows another inductance tuning arrangement;

Figure 6 shows a modified arrangement;` I Y inductance tuning Figure '7shows a radio circuit employing linductance tuning; and

Figure 8V shows -a-modied radio circuit.`

While a preferred embodiment of the invention is described herein, itisfcontemplated that-considerable variation Ymay -be made in the methodof procedure andthe construction of parts without departing fromthespirit of the invention. In the following description andV inthe claims,parts will be identified byl'specic names foi1 convenience, but they areintended to be as generic in their application to simila will permit.

In the drawings, Figure 1 represents rparts as the art a slide wirecurrently herewith.

A series, arrangement of iixe d end .inductance 4 (Cl. Z50-40) Y andfixed condenser 5 is connected between slip terminals of condenser 5. Ithas been found i" that a tuning ratio of 10:1 v'is feasible with such anarrangement. quency endr limiting inductance as small as 1% of the totalinductance at the low frequency exy As this necessitates' a highfretreme, it is necessary to have the resistance of the end inductancelow in order to develop the needed voltage at and near the highfrequency extreme. The end inductance might comprise a certain number ofunused turns at the high frequency end of the variable coil I, but it ispreferable to use a separate component in series with the variable coilunit.V

One advantage of this arrangement' resides vin the fact that the entirevariable coil may be utilized for varying the inductance. In additionthis arrangement has the advantage of enabling the use of an endinductance of better characteristics than an equivalent part of theunused turns on the variable coil, for the reason that under v practicaldesign limitations the form factor and losses of such unused end turnsof the variable unit are worse than those that can be obtained in apractical design of a separate coil. It will thus be apparent` that, bythe arrangements described,

the high frequency resistance of end inductance 4 Tis less than theresistance of a portion of the variable inductance l of equal inductivevalue or, in other Words, the ratio of the reactance to the resistancefor coil 4 is greater than for such part of coil l as would be requiredin the circuit if coil 4 were not used. This ratio is commonly used todesignate the characteristics of an inductive coil. The ratio is usuallyexpressed mathematically as f of the coil combination obtained by usingaseparate end inductance instead of a number of unused turns at the endof the variable coil. The dotted curves 3l and 32 represent the value ofQ over two frequency ranges for a given variable coil withl proper endinductances 4 and capacitances 5 in each range. The solid curves 33 and34 show the Q of the variable coil over these ranges with the samecapacities as used for curves 3l and 32, respectively, but without theuse of the separate end inductances 4. It follows that the tuning rangefor a variable inductance and xed capacity may be increased by the useof the end inductance, because the resista-nce of the circuit in theregion of the high frequency extreme has been lowered and hence thelimit Where an acceptable value of impedance is obtainable has beenmoved out to a higher frequency.

The increased frequency range made po-ssible by this tuning systemintroduces another problem in the design of tuning circuits. This iswhere the variable inductances are employed as pre-selectors insuperheterodynes. The pre-selector consists of one or more resonantcircuits tunable to the frequency of the incoming signal and anothertunable resonant circuit determining the frequency of the localoscillator. It is customary in superheterodynes always to maintain theoscillato-r at a frequency differing from the tunable incoming signalresonant circuits by an approximately constant amount called theintermediate frequency. 'Ihe method or procedure 'of maintaining thisfrequency difference during tuning is usually spoken of as tracking,

The methods employed to track variable condenser tuning circuits andvariable inductance tuning circuits in superheterodynes will be betterunderstood by reference to Figures 3, 4, 5 and 6.

Figure 3 shows the customary method of tracking variable condenseroperated pre-selectors in superheterodyne circuits. The variablecapacities 8 and 9 are electrically Yand mechanically similar andmounted on a common shaft. The radio frequency circuit comprisesvariable capacity 6, minimum frequency adjusting capacity 1, and xedinductance 8. In order to track the oscillator over the requiredrestricted range, fixed capacity II is placed in series with variablecapacity 8 in the oscillator circuit and fixed inductance I2 in theoscillator circuit is given a smaller value than fixed inductance 8 inthe radio frequency circuit. A minimum adjusting capacity IU isconnected in shunt with variable capacity 9.

An equivalent pair of circuits for pre-selection employingvariableinductances is shown in Figure 4. The radio frequency circuithere comprises variable inductance I4, adjustable minimum frequencyinductance I5 (to be referred to as the end inductance) and adjustablefixed capacity I3. The restricted range oscillator circuit consists ofvariable inductance I'I, adjustable minimum frequency end inductance I8and adjustable fixed capacity I5. In addition, there is the inductanceI9 connected in shunt with the capacity I6. 'I'he variable inductancesI'I and I4 are similar electrically and mechanically and are on a commoncontrol shaft for tuning purposes.

The variable capacity method of Figure 3 and the variable inductivetuning method of Figure 4 will yield similar tracking characteristics.With either of these methods the frequency difference between the radiofrequency and oscillator circuits may be made exactly equal to theinte-rmediate frequency at three different points over la tuning rangeand by proper design and adjustment these points of exact tracking maybe so positioned in the tuning range that the deviation from perfecttracking between these points will be at a practical minimum for thisarrangement.`

This condition exists when the maximum deviation between points and atthe ends of the tuning range is about equal.

By making end inductances I5 `and I8 as. separate components similar toinductance l in Figure 1, an increased tuning range is made possible butthe increased tuning range makes the tracking points more widely spacedthus resulting in increased deviation from perfect tracking betweenpoints. An additional tracking correction is therefore desirable in sucha system adapted for use for an expanded tuning range.

Figure 5 shows an improvement over the tuning circuit of Figure 4 wherein the ganged variable coils 28 and 2l of the type indicated in Figure land set forth in detail in my above-mentioned co-pending applicationshave been added, together with a mutual coupling coil 22 coupled to thevariable coil 2l.

I have found that if the shunt inductance I9 shown in Figure 4 is movedso as to have an inductive relationship with variable coil II thetracking may be materially improved. As shown in Figure this isaccomplished by dividing the shunt inductance into two parts, 22 and 23,and disposing the part 22 concentrically inside or outside of variablecoil 2I so that it will inductively couple therewith. It will be notedfrom Figure 5 that the mutual inductance between coils 22 and 2i willvary with the tuning because as the moving contacter is moved upwardless and less turns of coil 2l will be in the circuit, the part of thevariable inductance 2l below the contactor being short circuited asshown. The coils 22 and 23 constitute a shunt around the parallelresonant circuit. polarity causes the inductance of the whole circuitt'o be reduced instead of increased, and suitable physical conditionsare provided, such as the size and location of coil 22 at its optimumposition with respect to variable coil 2| and the optimum selection ofthe sizes of coil 23, end coil I8 and fixed capacity I6, then thetracking is found to be greatly improved in a wide range circuit overthat which is possible with the three point arrangement of Figure 4.that the adjustment of components of the oscillator circuit of Figure 5are made with a given set of component adjustments of its cooperatingradio frequency circuit and with a predetermined intermediate frequency.

Figure 6 shows a modification of the circuit of Figure 5 whereinindividual adjusting capacities 25 and 21 are shunted around Xed endcoils 24 and 26, respectively, in place of the directly adjustable andsimilarly disposed end coils I5 and ,f

I8 of Figures 4 and 5. This modification permits the use of standardcommercial adjustable trimmer condensers and other standard parts.

Figure '7 shows a complete pre-selection system for a superheterodyneradio receiving set employing three variable inductance tuners of thetype indicated in Figure 1 and described in my copending cases,mechanically operating in unison on a single shaft (i. e., ganged), thusaffording single control for tuning. The tube 88 is a radio frequencyamplifier, the tube 82 is a modulator and tube 87 is an oscillator tube.One of the inductive tuning units 6I is employed to tune the circuit ofthe antenna 50. A second variable inductance unit 'I6 is employed fortuning the output of tube 66 (which is also the input of tube 82) atradio frequency. 'I'he third inductive tuning unit 88 is for the purposeof maintaining the frequency of the circuit of oscillator tube 81 at asubstantially xed frequency spacing from the If coil 22 is connected sothat its It is to be understood ,1

v:frequency"ofthe two other radio frequency cirvto the'xediintermediatefrequency amplifier by circuit 91. Circuit 91 is the primary circuit ofthe rst intermediate frequency transformer of a Vstandardsuperheterodynecircuit. There may be Vone or more such 'transformers tuned to theintermediate frequency; The intermediate frequency circuit andi theaudio frequency output circuit may be of standard design.

Anode power for the` modulator tube 82 is supplied from' the battery |99connected to the lower part of circuit^91 as shown.

There are a number of single pole two-position switches :disposedthroughout the pre-selector and it is understoodthat all of theseswitches are mechanically attached to a common actuating means so thattheyI areeither all simultaneously snapped tov position L or to positionH. Position L connects'all of the A'components so as to operate overaflow frequency :wave range which may cover .54 tov2.50"rnc/s."(megacycl`es per second) and when 'invthe position H theYcircuit components are ofisuchsize andconnection as to en abletuning'thasystemover a high frequency 'range which maybe adjusted tocover 2.30 to 18.00 .mc/S. Y u

Consideringv the antenna tuning circuit which is connected to the inputterminals of radio fre-l quency amplifier tube-Bgwhen the switches are*in the "lowf frequency position L signal current entering" the systemby way' of antenna. 5U is divided just tothe'fleft of :switch |60 andpart of the currentpasses-throughcapacitance 5| and `chol'ec'oil53to-g'ro'und'. The choke coil 53 is designed--toresonate at afrequencyrbelow the ilowerlimitofthe low'frequency range. Currentalsofpasseswin thev other Vbranch "of the divided antennacircuit'througlr'capacitanjce 52 to a junction betweenfcapacitance*55andgrounded capacitance 55 whi'ch'ftogetherlform the resonant circuitiiixed capacitancel for f the low frequency range.f lTheinduct'anc'eofthis range comprises `adjosta-ble end inductance 54 andthe variable ini VV'The' steppedLup'voltage developed V'across the tunedimpedance of this resonant circuit is then Lled by'wayAof switch |00andlcapacitance 62 to 'the control vgrid of tuba-66 and byway of groundVand capacity 65 to "the cathode of tube 59. "Throughout most of thisrange-coupling from the vantenna is supplied -through the branchcontainingucapacity 52' and nearthej low frequency end of this rangeyresonantfchoke 53;,coupled to end ance'll are connected acrosscapacities58 and 59.- The voltage-across'the tunable impedance thus formed is ledthrough' switch 109 and capacity 62 tothe control gridof tube 65 and byway .of ground/and Vcapacity 6'5`to the cathodey of tube `96 asypreviously described. Resistance 63,- high compared with theresonantcircuit` impedance, ris for the purposeof 'supplying automaticvolumecontrol voltageinthe usual manner across Athe input terminals oftube '66.- Capacity 52 is for passing ra'diofrequency and preventing theshort. circuiting of the automaticvolume lcontrol 16, and for thehigh-frequency range capacitance 14, end linductance 15 and likewisevariable inductance 16. `The choke' coil 1|-is effectively connectedacross the output terminals of tube 66 and is employed to supplythattube with anode power from the battery |99; The characteristics of thechoke 1 are such' as to afford the highest impedance over the entiretuning range of the system Without causing resonance at any frequencywithin that range. The' capacitance 1li s for the purpose 'of passing'radio'frequency current and blocking the D..C. current' from battery|99 from being short-circuited by thetuning components. Capacitance andkresista-nce '18' are employed in' the same manner for tube 82respectively as capacitance 62 and'resistance 63 are in the case of tube66. The path ofthe radio frequency current betweeny tubes 66 andf'82 isfrom the 'anode of tube` 66 by way of capacitances 10 and 11 to thecontrol grid of tube 82 as shown, Vwith the tunable impedanceinterposedbetween this line and ground as shown. AThis variable impedance which istuned in unison with the antenna input "circuit already describedaccomplishes two purductance 88, end inductanc'e* 92 and xed'capacitance9|. In shunt witlrl'iixed` capacity 9| is the shunt inductance fdividedinto two parts 89 vand |5|. The part of the 'sl'ufnt inductance |6|isthe equivalent offc'oilf2'2'in Figures 6 and 5. When theswitches-arein thehigh frequency position H the tunable resonant;circuit of the oscilf lator comprises the' same' variable inductance 88,

the end inductance93 land 'fthe 'fixed capacity 94, in Vshunt with which-isthe shunt in-ductance 95. The feed-back or tickler circuit for thelow frequency range rimsfromithe anode of tube 81 through the switchland the tickler coil 90 to the positive pole of battery |99, and for thehigh frequencyk range fromfthe "anode through' tickler inductance 96tothe-positive pole of the same Ibattery. vOnthelow frequencyrange thepolarity ofY coupling between coils 99 andv|39 is such as vto produce-oscillationsand'likewise on the high frequency. rangev the'coupling-between coils- 96 and 19'5 is of` suchpolarity-as to1 produceoscillations; 'The resonantjtuning circuits kjust de- 'scribed areconnectedV by. way of fswitchrv |05 throughfcapacit'an'ce 85 toV-the'c'ont'i'pl gri-d of tubee1 anditheresistance 89 is-for the purposeof producing' `a suitable "bias. They oscillations thus. developed bythe4 circuits in cooperation' with tube 81 vare-supplied by wayofcapacitance 84 to the oscillator grid'of" modulator 'tube 82. Theresistance 89 is for the purpose of additionally adjusting the biasoftube 82 as the strength of the oscillatoryoltag'e may vary. f

Tube 92 maybe one of the well known pentavgrid types suchas 6A'1 or6A8'wherein theusecond vgrid out from thefcathode is known as the anodegrid. The connection shownv in Figure 7 where fthis grid is connectedto2-the' cathodev is not unusual, and enables retention of the desirableconversion gain properties of the pentagrid with the use of a separateoscillator tube as 81.

The two radio frequency tuning circuits employed in Figure 7 are for thepurpose of affording a much better superheterodyne image ratio thancould be obtained with a single radio frequency pre-selection circuit.Two important advantages obtained with a variable inductance method overthe customary condenser tuned method are considerably wider tuning rangeper switch position, and improved radio frequency amplification on theshorter waves. The Wider tuning range ma-de possible with this systemresults in greater economy due to the reduction in the number ofcircuits required, and also enables the use of considerably longer shortwave dial scales with greater accuracy than is possible with presentcondenser tuned systems.

An important advantage of inductive tuning especially where widefrequency tuning ranges may be covered, .as with this system, is that asubstantial amount of xed capacitance remains in the circuit at alltimes. At the high frequency en-d of a range Where condenser tuning isemployed, a large part of the total resonant circuit capacitance is dueto the wiring and input or output electrodes of the yassociated tubes.On the other hand with inductive tuning a much less proportion of thexed capacitance is made up of the circuit and tube capacitances.

In certain applications it may be desirable to employ a shunt coilacross the xed capacity of the radio frequency circuit, as across thecapacitance I3 of Figure 4 as Well as the xed inductance I9 across thexed capacity |6 of the oscillator circuit.

It has been found that the presence of the choke coil 1| in the outputof tube 66 of Figure '7 has aided the tracking of this stage with theantenna input tuning stage tunable by variable inductance 6|. This isbecause the slight frequency range restricting effect of choke 1| aboutequals the restricting effect of the antenna circuit by coupling to thetunable resonant circuit.

Figure 8 shows a superheterodyne pre-selector employing two variableinductance units |29 and |50. These variable inductances are 'similarelectrically and mechanically and are mounted on a common shaft forsingle control of tuning.

These variable inductance units are individually shielded as in the caseof the three variable inductance units shown in Figure 7.

The radio frequency signalsenter the pre-selector through antenna V||0and pass- ,through wave trap This trap is resonant to-the intermediatefrequency and is customarily employedin single pre-selection circuits toprevent reception of signals in the region of this frequency. On the lowfrequency range all of the switches are in the position L and theantenna `signals for this range pass through'switch ||2 and are dividedthrough capacitances ||1 and H8. Those passing through capacitance ||1go to ground through coil |26 which is designed to resonate below thelow frequency limit of this range. Those passing through capacitance I8go through inductance ||9 to the joint between series capacitances |24and |25, capacitance |24 being connected to ground by switch |30 on thisrange. It will be observed that the capacitances |24 and |25 in seriesconstitute the resonant circuit flxed capacitance for the low frequencyrange. The remainder of the low frequency resonant circuit is throughswitch ||6 and end inductances` |28, |21 in series and the variableinductance unit |29.

The size of inductance ||9 is chosen so that the antenna circuiteffective capacitance in conjunction with capacitance ||8 andcapacitance |24 will resonate outside the high frequency limit of thelow frequency range. The coupling of the antenna circuit to the inputresonant circuit over the low frequency range is by mutual inductancebetween coils |26 and |28, and by the common capacitance |24.

On the high frequency range all of the switches are in the position Hand the operation of the antenna input circuit is as follows: Fromantenna ||0 and through trap and switch H2, signals are fed throughcapacitance ||3 to the joint between capacitances ||4 and. H5, which inseries constitute the fixed capacitance of the resonant circuit for thisrange. In shunt with capacitances ||4 and ||5 is the end inductance |21(end inductance |28 being shorted out on this range by switch H6) andthe variable inductance |29. The coupling between the antenna circuitand the input resonant circuit on the high frequency range is by commoncapacitance ||5.

The resistance |32 is for conveying automatic Volume control voltage tothe control grid of tube |33, and the capacitance |3| is for passingradio frequency current and preventing the short circuiting of theautomatic volume control voltage by the tuning components.

'Ihe oscillator tuning circuit operates over the low frequency range byVariable inductance |50, end inductance |42 and the xed capacitance |44.'Ihe oscillatoi1 circuits are arranged to run at a higher frequency thanthe radio frequency circuits. To restrict its range a shunt inductanceis connected across and is made a part of the low frequency resonantcircuit. This shunt inductance is divided into two parts, coil |5| andthe mutual coil |60. The mutual coupling between coil |60 and thevariable inductance |50 varies with the tuning in the manner explainedin the description relating to the coil- 22 in Figs. 5 and 6.

The oscillator high frequency range resonant 'circuit is by the samevehicle inductance |50 and by end inductance |43 and fixed capacitance|45. The oscillator on this range also runs at a higher frequency thanits cooperating radioV frequency circuit, and for this purpose its rangeis restricted by the shunt inductance |46.

The tube |33 may be'one of the well known pentagrid types 6A7 or 6A8wherein the first and Ysecond grid from the cathode are respectively theoscillator grid and oscillator anode.

'I'he feed back orV tickler circuit for the oscillator low frequencyrange is from this anode grid through coils |41 and |52'and resistance|39 to positive terminal of battery |31, capacitances |53 and |38 beingfor by-passing purposes. The oscillator high frequency range feed backcircuit is the same as that used on the low frequency range exceptingthat the coil |52 is effectively shorted to ground by way of by-passingcapacitance |49 when the switch |48 is in the position I-I. The polarityof mutual coupling between coils |41 and |46 and also between coils I 52and |5| is such as to produce'oscillations. 'Ihe mutual coil |60 isshorted Vto ground on the high frequency range by switch |30.

The resistance |34 and the capacitance |35 are connected as shownrandarefor the purpose of producing the desired oscillator grid bias in tube|33. The resistance |32 -is for the purpose of conveying automaticvolume control voltage to thecontrolgrid of tube- |33 and thecapacitance |3| passes'radio frequency current but prevents theshortcircuiting to ground of the automatic volume control voltage by way ofthe tuning components.

of tube |33 are connected to a tap on the battery |31 as shown.

The coil |20 which is coupled to coil |I9 is connected between thecathode biasing resistor |2| and the cathode of tube |33, resistance |2|being by-passed by capacitance |227. The `lcoil |720 is employed only onthe low frequency range and its size and degree of mutual coupling withinductance ||9 as well as its positioning along the coil ||9 are such asto improve the superheterodyne image ratio of the circuit. This isdesir- '1 able where a single pre-selector circuit is used as in Fig. 8,and it may also be employed in a double pre-selector, as Fig. '1, althoit is not shown in Fig. 7.

The several variable inductance units shown in connection with thecircuits of Fig. 1 and Fig. 8 are individually shielded to preventpick-up of undesired signals and noise as well as to protect them fromatmospheric dust and dirt. The circuits just described could be extendedto cover more than two frequency ranges if desired.

The coil size for the variable inductances is limited by the highestfrequency to which it is desired to tune and must be such as to ensurethat the tuning range does not pass over its natural frequency. At thesame time it is desirable to make the coil as large as possible in orderto have the average impedance high over the tuning range. If any turnsare in the circuit while tuning through the natural frequency, there isusually enough mutual inductance between these turns and the shortedunused part of the coil to cause a serious loss in performance at andnear this point, and for this reason the coil geometry and number ofturns are chosen so asto have this natural frequency fall just outsidethe high frequency limit of the highest frequency range to be tuned.

There are methods of tuning through this natural frequency point, andthese are described in my co-pending application Serial No. 95,332,filed August 11, 1936.

In operating over the two ranges with the same variable inductances by4switching between two sets of suitable circuit components andarrangements, it has been found desirable to divide the total frequencycoverage so that less range is I covered in the low frequency than inthe high frequency switch positions. This is'because the considerablylarger fixed capacity required to tune the low frequency range rendersthe average inductance the greater the advantage described in connectionwith the curves of Fig. 2. y

It will be observed in Fig. 8 that the tracking aid coil |60 (andlikewise coil |6|.in Fig. '1) is in circuit only on the low frequencyrange as it is not foundto be necessary onthe high frequency range. Ingeneral tracking errors are less the farther removed the tuning range isfrom the fixed intermediate frequency, and the narrower the tuningrange. quency of coil |60 (and coil IBI) must be such when shorted outon the high frequency range by switch |30 in Fig.` 8 (or switch. |03 inFig. '1) that it is outside the limits of the high frequency range.

Unlike condenser tuning where the impedance of the circuit tends to belower at the lower frequency end of a tuning range, the variableinductance offers an additional advantage in the ease of obtaininguniform oscillations over a given range. 'Ihe increasing impedance asthe circuit is tuned toward the lower frequency end of a range with thisinductive tuner enables the use of a `high frequency oscillator feedback circuit whose natural frequency is outside the high frequencyrange, such a degree of feed back being sufficient to produce stableoscillations over a much wider range than would be possible withoutadditional feed back aids in condenser tuned circuits. The naturalfrequency of the feed back circuit is usually determined not only by thefeed back coil itself but also by its mutual coupling to any othercircuit plus the additional inductance and capacitance ofthe connectingleads and associated tube. The high frequency feed back coils justdescribed are shown as |41 and 96 respectively in Figs. 8 and '1. Stableoscillations are readily obtainable with the system just described overa frequency range of over 10 to l which requires a reactive ratio ofover 100 to 1. The feed back or tickler coil may be partly or whollyarranged inside the variable coil as outlined in my co-pendingapplication Serial No. 31,823, or may be coupled to the shunt inductance|46 by coil |41 as shown in Fig. 8.

This applicationy is a 'continuation in part ofl my co-pendingapplication for Radio apparatus, Serial No. 31,823, filed July 1'1,1935.

While the present invention, as to its objects and advantages, has beendescribed herein as carried out in specic embodiments thereof, it is notdesired to be limited thereby but it is intended to cover the inventionbroadly within the spirit and scope of the appended claims.

What is claimed is:

1. A resonant circuit for tuning within a predetermined frequency rangecomprising a slide wire variable inductance, a fixed inductance inseries therewith, whereby the magnitude of said variable inductance lmaybe varied between its maximum value and substantially zero during tuningover said predetermined frequency range, and a capacitance bridgedacross the series arrangement, the Q of said fixed inductance at a givenfrequency within said frequency range being higher than the Q of saidVariable inductance at said frequency when said variable in- The naturalfreductance is adjusted to an inductive magnitude K L being theinductance and `R being the high frequency resistance of the inductanceelement or portion of the inductance element in question andf being thefrequency.

2. A resonant circuit for tuning within a predetermined frequency rangecomprisingra varable inductance and a fixed inductance in series, and acapacitance bridged across the series arrangement, the Q of said fixedinductance at a given frequency within said frequency range being higherthan the Q of said variable inductance at said frequency when saidvariable inductance is adjusted to an inductive magnitude equal to thatof said fixed inductance, where L being the inductance and R being thehigh frequency resistance of the inductance element or portion of theinductance element in question and f being the frequency.

3. A resonant radio circuit for tuning within a predetermined frequencyrange comprising a variable inductance capable of reduction to sub1-stantially zero inductive value during tuning and a xed inductance inseries therewith, and a fixed capacitance bridged across said seriescombination of nductances, the radio frequency resistance of said fixedinductance at a given frequency within said frequency range being lessthan the radio frequency resistance of said variable inductance at saidfrequency when the variable inductance is adjusted to an inductivemagnitude equal to that of said xed inductance, whereby to improve theimpedance of the said resonant circuit in the high frequency regionWithin the said predetermined tuning range.

4. A resonant circuit for tuning within a predetermined frequency rangecomprising a slide wire variable inductance, a fixed inductance inseries therewith whereby the magnitude of said variable inductance maybe varied between its maximum value and substantially zero during tuningover said predetermined frequency range, a capacitance bridged acrossthe series arrangement, and an adjustable capacitance in shunt with saidxed' inductance whereby to enable adjustment of the high frequency limitof said tuning range.

5. A resonant radio circuit for tuning within a predetermined frequencyrange comprising a variable inductance capable of reduction tosubstantially zero inductive value during tuning and a fixed inductancein series therewith, and a fixed capacitance bridged across said seriescombina tion of inductances, the ratio of reactance to radio frequencyresistance of said fixed inductance at a given frequency within saidfrequency range being higher than the ratio of reactance to radiofrequency resistance of said variable inductance at said frequency whenthe variable inductance is` adjusted to an inductive value approximatelyequal to that of said fixed induct-V ance, whereby to improve theimpedance of the said resonant circuit throughout. at least the highfrequency half of the said predetermined tuning range.

PAUL WARE.

